1. Field of the Invention
The present invention relates to a positioning control system having a disturbance observer for estimating a disturbance from at least one observed value of a controlled object and feeding back an estimated disturbance. The present invention also relates to a filter that is used in such a positioning control system.
2. Description of the Related Art
Positioning control systems are widely used in production facility equipment. In clean room environments, particularly, pneumatic cylinders employed by positioning control systems are highly advantageous, since they are free of liquid leakage and can easily be serviced for maintenance.
However, pneumatic cylinders do not lend themselves to highly accurate and highly rigid positioning owing to their behavior, on account of air compressibility and limitations on the pneumatic pressure that can be used. In particular, a pneumatic cylinder having a seal mounted on a piston, for preventing leakage of air and entry of external foreign matter, is unable to perform highly accurate positioning due to a so-called stick slip phenomenon caused by frictional forces of the seal.
Japanese Laid-Open Patent Publication No. 2004-144196 discloses a positioning system comprising a pneumatic cylinder, which includes a piston incorporating a static pressure bearing in order to reduce frictional forces and provide increased positioning accuracy.
For increased control accuracy, some applications employ a disturbance observer for estimating a disturbance from at least one observed value of a controlled object and feeding back the estimated disturbance. For example, Japanese Laid-Open Patent Publication No. 2000-347738 reveals a positioning system employing a variable-gain disturbance observer for stably controlling an object irrespective of whether static friction or dynamic friction is involved in moving the object.
It is known in the art that it is effective to increase a control gain in order to increase positioning control accuracy. However, in order to protect the actuator, it is necessary to provide a suitable saturating element for preventing the command values from becoming too large.
The viscoelastic characteristics due to frictional forces of a seal in a pneumatic cylinder and the effect of such characteristics on control performance are described in “Viscoelasticity in Displacement of a Pneumatic Cylinder and its Effect on Control Performance,” by Osamu Oyama et al., a collection of articles of the Japan Fluid Power System Society, November 1998, Vol. 29, No. 7, pp. 19(155)-25(161). The article shows that the viscoelasticity of a seal is effective for stabilizing piston displacement more than when the piston slides, and it is possible, using this effect, to increase control sensitivity and thereby increase settling speed, so that displacement of the piston can be stabilized as a result of such viscoelasticity.
The positioning system disclosed in Japanese Laid-Open Patent Publication No. 2004-144196 is complex in structure and highly expensive, since it employs a static pressure bearing. Since the static pressure bearing causes too small an amount of friction, it has poor damping characteristics, tending to increase the convergence time for positioning.
The disturbance observer employed by the positioning system disclosed in Japanese Laid-Open Patent Publication No. 2000-347738 generally includes a low-pass filter, and exhibits integrator characteristics in a low frequency range. It is also known in the art that if a control system includes an integrator, then it suffers from a phenomenon known as integrator windup.
Integrator windup is a situation where, when a given positive deviation is steadily applied, the positive deviation is integrated excessively beyond a saturated value L of the manipulated variable applied to a controlled object, as shown in FIG. 35 of the accompanying drawings. If a negative deviation is steadily applied after time T1, then it is desirable under normal circumstances for the manipulated variable to be reduced immediately from time T1, as indicated by the broken-line curve 1. However, because of windup, the manipulated variable starts being reduced from an integral S, at time T1, as indicated by the solid-line curve 2, and the controlled object actually changes its operation from time T2. Therefore, the manipulated variable remains saturated at the saturated value L from time T1 to time T2. The controlled object thus suffers a delay in its operation, tending to result in a reduction in its control performance, e.g., increased overshooting. In order to protect the control system against integrator windup, the integrator may have a function to stop its integrating action beyond the saturated value L.
A simple integrator may relatively easily be designed to incorporate a means for stopping its integrating action beyond the saturated value L. However, no attempts have heretofore been made to add such a means to a disturbance observer, and to determine a location where such a means is provided with respect to a disturbance observer. Hence, it has not been possible to prevent integrator windup in disturbance observers.